A magnetic steel sheet has been used as a soft magnetic material utilized for an iron core of a static apparatus such as a transformer, a choke coil, and an inverter. However, a dust core is investigated as an alternative material of the magnetic steel sheet.
In general, the waveform of a current applied to a coil of a static apparatus includes a direct-current component together with an alternating-current component. When a DC current increases, the inductance of the coil decreases. As a result, the impedance decreases, thereby causing a problem in that, for example, an output decreases or a power conversion efficiency drops. Therefore, a soft magnetic material used for a static apparatus is required to have characteristics such as a low inductance drop with an increase in a DC current, that is, excellent DC bias characteristics and a low loss (low iron loss).
However, dust cores are inferior to magnetic steel sheets in terms of DC bias characteristics. This is because an inductance drop with an increase in a DC current is caused by magnetic saturation of soft magnetic materials. Specifically, the magnetic field applied to soft magnetic materials becomes large with increasing DC current. Consequently, magnetic saturation decreases magnet permeability. Since inductance is proportional to magnetic permeability, inductance drops.
To improve the DC bias characteristics of dust cores, a method for manufacturing a core and the core are disclosed in Japanese Unexamined Patent Application Publication No. 2004-319652 (Patent Document 1). Patent Document 1 discloses that an irregular soft magnetic powder having a particle size of 5 to 70 μm is used.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-319652